expint: Exponential Integral and Incomplete Gamma Function

The exponential integrals E_1(x), E_2(x), E_n(x) and
Ei(x), and the incomplete gamma function G(a, x) defined for
negative values of its first argument. The package also gives easy
access to the underlying C routines through an API; see the package
vignette for details. A test package included in sub-directory
example_API provides an implementation. C routines derived from the
GNU Scientific Library <https://www.gnu.org/software/gsl/>.